Apparatus for the electrolytic production of fluorine



y 27, 1954 A. J. RUDGE ET AL 2,684,940

APPARATUS FOR THE ELECTROLYTIC PRODUCTION OF FLUORINE Filed July 20,1950 INVENTORS III IIII IIIIII I I /III IIII I IIIIIIIIIIII IIIIIII IIIIII I IIIIIII II I n I I v v. I III III I I I IL a J-il l rWWII IIW-PkII I II I II IIIIIIII III III I IIIII I v x Na 1 MM ATTORNEYSPatented July 27, 1954 APPARATUS FOR- THE ELECTROLYTIC PRODUCTION OFFLUORINE Alfred John Budge and Harold Hill, Runcorn, and William NormanHowell, Widnes, England, as-

signors to Imperial Chemical Industries Limlted, a corporation of GreatBritain Application July 20, 1950, Serial No. 174,834

Claims priority, application Great Britain August 2, 1949 4 Claims. 01.204-247) This invention relates to improvements in electrolyticapparatus and processes in particular to apparatus and processes for theelectrolytic production of fluorine.

It is well-known to manufacture fluorine by electrolysis ofsubstantially dry fused mixtures of fluorides, in particular mixtures ofan alkali metal fluoride and hydrogen fluoride. Further, in such aprocess, it is well-known to use anodes of carbon or graphite, thecathode being of mild steel orother metal resistant to the action of theelectrolyte. Hydrogen is evolved at the cathode and fluorine, withvarying amounts of oxygen and other impurities, at the anode. Mixturesof hydrogen and fluorine give rise to violent explosions, therefore, anessential feature of all such fluorine cells is a diaphragm or partitionhaving as its function the prevention of mixing of the gases evolved atthe tWo electrodes. diaphragm or partition extends downward in theinterelectrode space for a distance equal to or even greater than thatof the downward extension of the electrodes. In other fluorine cells,for example in that described in British patent specification No.668,465, the separating diaphragm or partition extends downward for ashort distance only into the interelectrode space. It will be apparent,therefore, that a disadvantage of all In some cells this fluorine cellsof the type above described is that the interelectrode distance must besufficiently great to accommodate the gas-separating diaphragm betweenthe electrodes and to prevent the hydrogen entering the anodecompartment; this results in a longer interelectrode path for theelectric current, and consequently greater power consumption, than wouldbe necessary if the diaphragm were absent from the interelectrode spaceand the cathode located much nearer to the anode. In practice theinterelectrode gap must not be less than a certain critical minimumwhich is determined by a number of factors including the depth of thecathode and the state of its surface as well as the current density.

In a communication to Nature (vol. 160, p. 504, October 11, 1947) theapplicants reported their discovery that, in the electrolysis ofpotassium fluoride-hydrogen fluoride mixtures using amorphous carbonanodes, the gaseous fluorine appears almost wholly at the junction ofthe anode With the electrolyte surface, if the current density ismaintained below a critical maximum which is determined by a number offactors. The applicants have further discovered that, even when such ananode has a horizontal part below the electrolyte, the fluorine is notliberated from this horizontal portion, but passes along it towards thevertical part of the anode.

In U. S. Patent No. 2,592,144 is described a diaphragmless fluorine cellin which the porous carbon anode is wholly immersed in the electrolytewhich does not enter the pores of the carbon. The fluorine liberated asa result of the electrolytic process passes inwards through the pores ofthe carbon and is conducted therefrom through a suitable conduit inflxedin the upper end of the immersed anode.

According to the present invention, a cell for the production offluorine by electrolysis of a liquid mixture of flourides includes thefollowing features.

The cell comprises a container for the electrolyte provided with acathode and a carbon anode. A heating jacket surrounds the container toa height of at least one half of the longitudinal length of saidcontainer. The top of the jacket defines the normal operational level ofelectrolyte. The upper portion of said carbon anode is of smaller crosssectional area and longitudinal length than the lower portion thereofand the said lesser upper portion is positioned above the level of thetop of the cathode and below the normal operational level of theelectrolyte.

A gas collecting hood of metal resistant to attack by the electrolyteand electrolytic products is positioned to surround only the said lesserupper portion of the anode without contacting the same. The downwardgeometrical projection of the edge of the gas collecting hood liessubstantially within the periphery of the major lower portion of theanode. In addition, the upper portion of the said hood is provided withan outlet for gas collected therein.

In the process according to the invention the fluorine liberated as aresult of the electrolysis passes mainly up the vertical surface of theanode and travels round the shoulder thereof and is discharged into thespace under the hood or hell. No appreciable amount of fluorine isliberated in the form of detached bubbles into the interelectrode space.A portion of the fluorine may find 3 its way into the bell through thepores in the mass of the carbon anode, the relative proportion of thegas following this route is, of course, dependent on the porosity of thecarbon, the surface roughness thereof, and probably on other factors.

Although electrolytes containing varying proportions of the metalfluoride and hydrogen fluoride may be employed in the present inventionwe prefer to employ electrolyte having composi tions lying within therange 1 KF/l.8 HF to l KF/2.2 HF and to carry out the electrolysis attemperatures within the range 80 C. to 110 C. If temperatures of theorder of 250 C. are employed the electrolyte should approximate incomposition to 1 KF/l HF.

Suitable cells according to the invention comprise a mild steelcontainer for the electrolyte which container may be cylindrical orrectangular as desired, provided with a jacket which is adapted for hotwater or steam heating, alternatively it may be heated by suitableelectrical means. The shape of cross-section of the anode is in no waycritical although it is generally convenient that it should be circular.At the region where the cross-section of the anode changes from that ofthe main portion to that of the upper part the transition from thegreater to the lesser anode dimensions may be effected by means of astep or shoulder, which step or shoulder may be horizontal or inclinedinwardly toward the vertical. On the other hand, the anode top may betapered so that it can be conveniently housed under the bell or hood.

In one preferred form of our invention we use anodes formed of porouscarbon or graphite having a permeability of more than 10, permeabilitybeing defined in terms of cubic feet of air per square foot of surfacecapable of passing through one inch thickness of the carbon per minuteunder an imposed pressure equivalent to two inches of water. However,the employment of anodes of high permeability is not essential for theproduction of fluorine according to the present invention, since quitesatisfactory results may be obtained with a relatively dense andimpervious carbon anode.

Using anodes as above described and working under the above specifiedconditions it is possible to operate fluorine cells with interelectrodegaps of inch or less without danger from intermingling of theelectrolytic gases and with considerable saving in energy consumption.

A cell suitable for carrying out the invention is shown in the attacheddrawing not drawn to scale, which represents a vertical section throughthe said cell. Referring to the drawing, l is a container of mild steelor other suitably resistant metal, provided with a lid 2, and a jacket 3adapted for water or steam heating surrounds the container l. The carbonanode 4, the upper portion of which is of narrower cross-section thanthe lower portion is partially submerged in the electrolyte 5. Anelectrically conducting rod '6 insulated from the cell lid 2 isconnected to the anode 4. At close proximity to the anode is a cathode lwhich may be of mild steel, copper or other material substantiallyresistant to the electrolyte and products of electrolysis. The cathodeis supported by an electrically-conducting rod 8 which rod is insulatedfrom the cell lid through which it passes. Surrounding the upper portionof the anode and which dips into the electrolyte is a hood 9. The pipeIt for fluorine take-ofi is connected through the cell 4 lid to thespace between the upper portion of the anode and the hell 9. The pipe llconnected through the cell lid is for take-off of hydrogen.

The following example illustrates but does not limit the invention.

Example The cell used in this example was of the shape described in thedrawing. The container of the cell was a jacketed mild steel vesselwhich was heated by hot water. The anode was cut from a block of hardcarbon having a very low permeability. The height and width ofcross-section of the lower portion of the anode was 9 inches and 3inches respectively; the height and width of cross-section of the upperportion of the anode was 8 inches and 2 inches respectively. Theanode/cathode separation was 0.48 inch. All the fluorine was evolved upthe vertical surface of the upper portion of the anode and was withdrawnbetween this part of the anode and the bell or hood which enclosed it.

The following measurements were noted when the cell voltage was 7 .6volts and the temperature of the electrolyte was C. to C.

Fluorine content of anode gas- 96.2% (rest mainly oxygen) By way ofcontrast when the run was repeated under similar conditions to the abovebut with an anode/cathode separation of 3 inches the voltage was 8.6volts.

Having now particularly described and ascertained the nature of ourinvention and in what manner the same is to be performed, we declarethat what we claim is:

l. A cell for the production of fluorine by electrolysis of a liquidmixture of fluorides which comprises a container for the electrolyteprovided with a cathode and a carbon anode, the upper portion of saidcarbon anode being of smaller cross-sectional area and longitudinallength than the lower portion thereof and said lesser upper portionbeing positioned above the level of the top of said cathode, and a gascollecting hood of metal resistant to attack by the electrolyte andelectrolytic products positioned to surround only the said lesser upperportion of said anode without contacting the same, the downwardgeometrical projection of the lowest edge of which hood liessubstantially within the periphery of the major lower portion of thesaid anode, and the upper portion of the said hood being provided withan outlet for .gas collected therein.

2. A cell as set forth in claim 1 wherein the anode is formed of carbonselected from the group consisting of amorphous carbon andgraphitizedcarbon of a permeability greater than 10, said permeability beingdefined in terms of cubic feet of air per square foot of surface capableof passing through one inch thickness of the anode material per minuteunder an imposed pressure equivalent to two inches of water.

3. A cell as set forth in claim 1 wherein said container is providedwith an external heating jacket.

4. A cell for the production of fluorine by electrolysis of a liquidmixture of fluorides which at least part of the said upper portionextending 10 below the said normal operational level of electrolyte andbeing positioned above the level of the top of said cathode, and a gascollecting hood of metal resistant to attack by the electrolyte andelectrolytic products positioned to surround only 15 the said lesserupper portion of said anode without contacting the same, the downwardgeometrical projection of the lowest edge of which hood liessubstantially within the periphery of the major lower portion of thesaid anode, and the 2 upper portion of the said hood being provided withan outlet for gas collected therein.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 862,783 Allen Aug. 6, 1907 1,484,733 Mathers Feb. 26, 19241,484,734 Mathers Feb. 26, 1924 2,034,458 Calcott et a1 Mar. 17, 19362,506,438 Whitaker May 2, 1950 OTHER REFERENCES Schumb, The Generationof Fluorine," Ofiice of Technical Services, publication PB 32205, June15, 1943, pages 10, 21 and 23.

Karr, Elemental Fluorine, Ofl'ice of Technical Services, publication P.B. 44659, June 15, 1946, pages 4, 19, 21 and 23.

1. A CELL FOR THE PRODUCTION OF FLUORINE BY ELECTROLYSIS OF A LIQUIDMIXTURE OF FLUORIDES WHICH COMPRISES A CONTAINER FOR THE ELECTROLYTEPROVIDED WITH A CATHODE AND A CARBON ANODE, THE UPPER PORTION OF SAIDCARBON ANODE BEING OF SMALLER CROSS-SECTIONAL AREA AND LONGITUDINALLENGTH THAN THE LOWER PORTION THEREOF AND SAID LESSER UPPER PORTIONBEING POSITIONED ABOVE THE LEVEL OF THE TOP OF SAID CATHODE, AND A GASCOLLECTING HOOD OF METAL RESISTANT TO ATTACK BY THE ELECTROLYTE ANDELECTROLYTIC PRODUCTS POSITIONED TO SURROUND ONLY THE SAID LESSER UPPERPORTION OF SAID ANODE WITHOUT CONTACTING TTHE SAME, THE DOWNWARDLYGEOMETRICAL PROJECTION OF THE LOWEST EDGE OF WHICH HOOD LIESSUBSTANTIALLY WITHIN THE PERIPHERY OF THE MAJOR LOWER PORTION OF THESAID ANODE, AND THE UPPER PORTION OF THE SAID HOOD BEING PROVIDED WITHAN OUTLET FOR GAS COLLECTED THEREIN.